Chronic infection with Toxoplasma gondii will often reactivate and cause life-threatening Toxoplasmic encephalitis in patients with AIDS or other immune deficiencies. The molecular basis of chronic infection with Toxoplasma gondii is the tissue cyst that develops and persists in the central nervous system. The elimination of cysts in infected individuals would prevent Toxoplasmic encephalitis, as well as deteriorating neurocognitive function that occurs in HIV-positive individuals even prior to AIDS. However, no drug or other therapy exists that can target the chronic Toxoplasma cyst. Currently, there is a deficit in fundamental biological knowledge regarding parasite and host biology that controls the development, maintenance, and reactivation of Toxoplasma cysts. We hypothesize that dense granule (GRA) proteins that associate with the intravacuolar network (IVN) membrane system play biological roles in manipulating the host CD8+ T cell responses that control chronic infection, as well as biological roles as structural, signaling, or sensing components of a membrane system that influences the development of cysts, the maintenance of cysts, and the reactivation of cysts. This hypothesis for a dual role for IVN associated GRA proteins in chronic infection is supported by our preliminary data. This exploratory R21 proposal will investigate the role of IVN GRAs in cyst development, maintenance and reactivation (Aim 1), and in manipulating the host CD8+ T cell responses that determine whether cysts are maintained, cleared, or reactivated (Aim 2). We anticipate these high impact studies will expand our fundamental knowledge of the biology that regulates cyst development, maintenance, and reactivation, and may therefore establish a molecular basis for an intervention that can eradicate chronic Toxoplasma gondii infection.